The present invention relates to liquid level detectors or indicators for liquid metals, or other conductive liquids, and particularly to such a detector which is inherently temperature compensated and which provides continuous liquid level indication without requiring calibration.
Various types of level detectors or gages for liquid metal are known. In the inductance type of level detector, a coil or coils are used which can be submerged in the liquid metal, as in a well or probe, for example. The presence of a conductive medium such as liquid metal immediately surrounding the coil changes the inductance of the coil, as compared to its inductance in air, and this change can be used to detect the liquid level by sensing the change in inductance, or the corresponding change in impedance. This may be done by actual measurement of the inductance or impedance but it is preferably done by comparing the impedance of a sensing coil with that of a reference coil which is substantially unaffected by the liquid metal.
In such a device, however, the inductances of both the sensing coil and the reference coil are affected by temperature, and possibly other environmental conditions, so that erroneous indications may be obtained unless some form of temperature compensation is provided. This may be done by comparing the sensing coil with a reference coil which is shielded so as not to be affected by the liquid metal and is subjected to the same environmental conditions as the sensing coil, so that temperature effects cancel out, as disclosed in a copending application of L. F. Marinaccio, Ser. No. 297,875, filed Oct. 16, l972, now Pat. No. 3,777,566 and assigned to the Assignee of the present invention. Such an arrangement is satisfactory for sensing a liquid level but is not well adapted for providing a continuous indication over the entire depth of a metal bath, and accurate calibration is required if such indication is needed. It has been proposed to provide such continuous indication by comparing each one of a series of sensing coils individually with a remote reference coil, as in Roeske et al U.S. Pat. No. 3,326,043, but serious errors may occur in such an arrangement because of temperature differences between the sensing coils and the reference coil and temperature compensation is not easily obtainable in such an arrangement.